The primary quality of a carbon black which determines its reinforcing ability is its surface area. Many methods have been developed over the years to measure the surface area. One analytical method which is used is nitrogen adsorption. In the nitrogen method the nitrogen is adsorbed onto the carbon black. The carbon black with the adsorbed nitrogen on its surface is then heated. The amount of nitrogen which is released is then measured and this amount of nitrogen is correlated to the surface area of the carbon black. One problem with the nitrogen gas test is that it measures the entire surface area. The nitrogen is adsorbed into the pores of the carbon black as well as on the gross surface. And although it is relatively fast, taking about 15 to 20 minutes, it is not useful for measuring the non-porous surface area of the carbon black. A modification of the nitrogen gas test is the nitrogen T-area analysis which uses nitrogen gas at different pressures in order to determine the surface area. However, this test takes several hours to perform.
Another analysis which has been developed to measure the surface area of carbon black is the iodine number test. This analysis is started with a definite amount of iodine, which is contacted with the carbon black. The iodine which has not been adsorbed is measured. This is a solution adsorption versus the gas adsorption used in the nitrogen method. This method is better than the nitrogen gas method in some respects because the iodine molecule which is adsorbed is a relatively large molecule and doesn't get into the pores (other than some of the medium sized pores). The iodine also has the advantage of being readily adsorbed by the carbon black. The method is also relatively easy to perform by simply dissolving the iodine in water and mixing it with the carbon black. The mixture is centrifuged, and the supernatant is separated and the amount of iodine measured by titration. There is a disadvantage with the iodine analysis, however, in that the iodine number is influenced by factors not related to surface area. If the carbon black has solvent-extractable impurities, these will affect the numbers. For example, the iodine number of a fluffy black versus pellets will be different, even though the carbon black used has the same non-porous surface area. If the black is oxidized, this will also affect how much iodine is adsorbed. The analysis is capable of being deceived by non-surface area factors. Therefore it is possible to get blacks with the same iodine number, but not the same non-porous surface area, or 2 blacks with different iodine numbers can actually have the same non-porous surface area, because of surface impurities.
An additional analysis which overcomes some of the disadvantages of the above methods is commonly referred to as the CTAB analysis. This analysis involves the use of a large organic molecule, cetyltrimethylammonium bromide (i.e. CTAB). This analysis is similar to the iodine number analysis in that the CTAB is adsorbed onto the surface of the carbon black. However, it is immune to non-surface area factors. On the other hand, it is a more time consuming method than the iodine number method and is subject to more inaccuracies for other reasons. Like the iodine number method this is a solution method where the CTAB is dissolved in water. The carbon black is placed in a bottle with a stirrer, the CTAB is added, and the mixture stirred to make a dispersion. The dispersion is then filtered and the filtrate collected. The amount of CTAB in the filtrate is then measured and based on this amount, the amount of CTAB adsorbed by the carbon black and the non-porous surface area of the carbon black are determined. Because of the amount of material handling which is required by this method, results of the CTAB analysis can often be erratic. The method is also very time consuming, with generally only about 4 samples per hour capable of being processed. While the CTAB analysis is better in some respects than the iodine number analysis, it is also more time consuming and prone to unreliablity.
Accordingly, what is needed in this art is an improved analysis method for determining the non-porous surface area of carbon black which overcomes the above problems.